It depends. If it's some sort of game where she's testing you and she'll be mad if you don't get her anything, you need to figure out some way of getting out of that sort of gameplaying because it's just unhealthy and destructive to a relationship. At least have a candid conversation about whether she really wants something and what she really wants from you and try to get your communications to where they are honest. It's just not good to be in a relationship where people aren't telling the truth about what they want.
If she's just a self-sacrificing person who really doesn't want anyone to go to any trouble over her, I would still get her something small but nice (nice, inexpensive jewelry is good if she likes that sort of thing).